1. Field of the Invention
The present invention relates generally to personal protective equipment and in particular to a personal respirator having a filter for filtering air during breathing.
2. Description of the Related Art
A filter of a conventional reusable filter respirator is generally in the form of a cylindrical canister in which impure air is drawn axially through the canister. The canister is generally connected to the face piece by a suitable connection device. Such connection device may incorporate an inhalation valve. Some respirators may be fitted with two or more filter canisters to increase the absorptive capacity of the filter system and to reduce the inhalation resistance. The overall depth of a complete respirator fitted with a single filter is therefore the sum of the depth of the canister, the depth of the connection device and the depth of the face piece. In a conventional half-face piece respirator fitted with a gas and vapor filter, the overall depth of the canister can be in the region of 50-75 mm and the overall depth of the complete filter respirator can be in the region of 100-120 mm.
This depth has a number of disadvantages: the center of gravity of the filter is a significant distance from the wearer's face, and therefore imposes a load on the wearer's neck muscles; the center of gravity of the filter is a significant distance from the center of rotation of the wearer's head (which is effectively above the spine), and therefore imposes a resistance to rotation, or cessation of rotation, of the wearer's head in both the horizontal and vertical planes; 3) the location of the center of gravity imposes a significant second moment of inertia, with the consequence that movement of the wearer's body can cause the respirator to “wobble” on the face, therefore affecting the efficiency of the seal between the respirator face piece; and, the respirator has significant bulk.
Where the respirator is intended for self-rescue from emergency situations, the bulk of a conventional respirator can result in a device which cannot conveniently be carried on the person; and may therefore not be available in an emergency situation when required. Also, if a large number of respirators have to be stored in case of emergencies, for example in a railway or underground carriage or subway, the bulk of a large number of stored respirators could affect passenger capacity and/or luggage capacity.
To provide protection in any emergency situation where a filter device for self-rescue could be used, i.e. a situation where oxygen deficiency is unlikely, a suitable device must be available to the person needing to escape. In setting performance requirements for a low-profile filter respirator for self-rescue it is therefore necessary to strike a balance between device weight and size on one hand and performance on the other so that it is likely that a suitable device will be available when required: if a device is too heavy or too bulky to carried on the person, in a handbag or briefcase, or too bulky to be stored in large numbers in public places such as schools, theatres or transport vehicles, it is unlikely to be available when required. A device that will protect all anticipated wearers under all foreseeable use situations may be so heavy and bulky that it is unlikely to be available: such a device may also be so complex to fit and wear that it may not be used correctly in an emergency situation. It may therefore be impracticable to attempt to provide adequate protection for all persons in all possible usage situations.
It is therefore considered that the performance requirements for a low-profile filter respirator for self rescue for use in non-occupational situations should be such that a complying device can be conveniently carried on the person, in a handbag or briefcase or securely stored in sufficient numbers in public places so as to be available when required.
It is considered that devices complying with EN 403 or DIN 58647-7 are likely to be too bulky and too heavy to meet the above criterion. For example, devices complying with either standard are permitted to weigh up to 1 kg.
The bulk and weight of devices incorporating gas filters essentially depend on the weight of the gas filter. For a given filter technology, filter weight can only be reduced by reducing the mass of absorbent/adsorbent and therefore reducing the mass of contaminant able to be retained by the filter.
Standard tests of gas filter capacity are based on test gas challenge concentrations which are intended to provide a relatively short test period, and thus a relatively inexpensive test, rather than on an assumption that the test gas challenge concentration will be experienced in practice in foreseeable use situations. For example, in EN 141 the minimum breakthrough period for a B1 filter is 25 minutes in a challenge concentration of 1,000 ppm of hydrogen cyanide. If it were assumed that the absolute capacity of a B1 filter does not change with challenge concentration, it is simpler and cheaper to test a filter at 1,000 ppm for 25 minutes rather than to test the same filter at 25 ppm for 1,000 minutes. This test is therefore not based on the assumption that a wearer of such a device is likely to be exposed to 1,000 ppm of hydrogen cyanide for 25 minutes. EN 141 states “The minimum breakthrough time is intended only for laboratory tests under standardized conditions. It does not give an indication of the possible service time in practical use”
It is therefore possible to design low mass gas filters for single use devices by specifying filter performance in terms of challenge concentrations to which the device is likely to be exposed over foreseeable self-rescue durations.